A method for conveniently measuring a protein concentration in an aqueous liquid sample containing a protein, such as urine (JP5 (1993)-249122A), is known. The method uses a pH indicating dye that exhibits a color transition when the dye reacts with a protein, thereby forming a protein indicating reagent. Such a protein indicating reagent has a color that also varies with pH, but changes in the color may correlate with protein concentrations when the pH is kept constant. Examples of such a protein indicating reagent include tetrabromophenol blue (TBPB) and tetrachlorophenol-3,4,5,6-tetrabromosulfophthalein. Since a change in pH causes the color of the protein indicating reagent to change, independently from the concentration of protein, the use of such a protein indicating reagent requires the use of a buffer in combination so as to keep pH constant.
(JP5 (1993)-249122A, U.S. Pat. No. 3,485,587).